1. Technical Field
This invention is directed to a method of substantially completely catalytically oxidizing methanol vapor and/or formaldehyde contained in a heated carrier gas and, more particularly, to a method of treating exhaust gases generated by burning a methanol fuel in an internal combustion engine.
2. Description of the Prior Art
As an alternative fuel for automotive vehicles, methanol (CH.sub.3 OH) has a high octane number that allows a spark-ignited internal combustion engine to operate at a high compression ratio for improved fuel economy. However, methanol fueled vehicles yield emissions of undesirable partial oxidation products, such as formaldehyde, as well as of unburned fuel, especially during the cold start portion of the driving cycle. Because of its potency as an eye irritant and as an ozone precursor, the formaldehyde emitted from methanol fueled vehicles must be efficiently reduced to a minimum.
Since methanol fueled engines of the future may be calibrated for a wide range of operations from stoichiometric conditions to lean burn conditions, the catalyst must be effective for complete oxidation of methanol over a wide range of methanol fuel mixtures supplied to the engine.
To date, the prior art has been concerned with either a lean or a stoichiometric engine calibration. In U.S. Pat. No. 4,304,761 there is described the preparation of an active silver (Ag) catalyst for the control of emissions from a methanol fueled vehicle, the engine of which is calibrated for lean burn conditions as exhibited by the test data of said patent. This patent concluded that the base metal silver alone would completely oxidize methanol at low concentrations, but only in the presence of excess oxygen (lean mixtures), to carbon dioxide and water vapor without production of deleterious amounts of aldehydes (3% or less), ethers, or carbon monoxide. More recent studies have shown that a 3% conversion of methanol, present in the exhaust gas, to aldehydes is not acceptable as a nonregulated automotive emission. The study in the said patent used a laboratory simulation of the exhaust gas from an engine that would require a lean air/fuel mixture to yield an exhaust gas containing 1% excess oxygen. The results of the study of both the treated silver catalyst and other catalysts for oxidizing methanol (all pretreated by heating at 800.degree. C. for six hours) were obtained at a space velocity of 300,000 Hr.sup.-1 and were based on results for granular catalytic support material (i.e., without monolithic substrate). This space velocity should not be compared with the space velocities applicable to comparable catalysts based on monolith substrates.
The results in the said patent show that certain catalysts, such as palladium and rhodium, did not function as well as the silver catalyst under the test conditions, and that patent reports that these materials have an undesirable affinity for producing aldehydes. There was no investigation of how silver, or any of the other catalysts employed, would perform in the presence of little or no oxygen or at conditions typical of exhaust gases issuing from an internal combustion engine.
In U.S. patent application Ser. No. 921,027 (assigned to the assignee of this invention), it is demonstrated that palladium, when supported on a monolithic ceramic substrate of a desirable cell density, is effective in converting methanol to CO.sub.2 and water vapor accompanied by little or no formaldehyde formation when the engine is calibrated for stoichiometric conditions.
Silver and palladium have not been used together in any specific separated sequence for treatment of exhaust gas containing alcohol vapor. The prior art, in the use of sequential treatment of exhaust gases, has been concerned only with the exhaust gases from a gasoline fueled vehicle, which gases are emitted at much higher temperatures. Such is exemPlified in (i) U.S. Pat. No. 3,503,715 which used two separate pellet bed catalysts, (ii) U.S. Pat. No. 3,896,616 which combined an extraneous fuel with gasoline exhaust gases at temperatures of 500-1400.degree. F. to reduce nitrogen oxides, hydrocarbons, and carbon monoxide to acceptable levels (each bed of which comprised the same catalyst), and (iii) U.S. Pat. No. 4,225,561 wherein the catalyst of the first bed was comprised of comingled silver and palladium and the catalyst of the second bed was comprised of chromium. None of these patents discuss the concept or need for reducing formaldehyde as a result of converting methanol fuel since gasoline was the only fuel involved.
Thus it remains a problem to be able to effectively oxidize residual alcohol vapor, particularly methanol, in the exhaust gas of an alcohol fueled vehicle to ensure that little aldehydes remain in the exhaust gas, regardless of whether the engine uses a rich, lean, or stoichiometric air/fuel mixture.